Hepatocellular carcinoma (HCC) is a major cause of cancer-related death worldwide with the fastest rising incidence of all cancers at present time. Sorafenib is an oral multikinase inhibitor that holds as the only consistently efficacious systemic therapeutic agent in the treatment of advanced staged HCC as demonstrated in recent Phase III clinical trials. Patient data reveals frequent resistance to sorafenib by means of either acquired resistance, in which the tumor progresses during treatment despite initial drug sensitivity, or de novo resistance, in which a tumor is refractory at the onset of treatment. Interestingly, TWIST1 expression also correlates with poor prognosis and advanced stages among HCC patients. TWIST1 also shows a correlation with resistance to other systemic therapies and my preliminary data suggests TWIST1 can confer sorafenib resistance to HCC cells. The precise pathways that TWIST1 may influence to cause sorafenib resistance have not been studied thus far. Preliminary data suggests that TWIST1 may be activated downstream of Signal Transducer and Activator of Transcription 3 (STAT3) and once activated may initiate a variety of signaling cascades to induce sorafenib resistance. Phosphoprotein RTK antibody array data suggests that PDGFR, HER3, and EphA3 may be upregulated by TWIST1 to mediate sorafenib resistance in HCC cell lines. As such, the mechanisms by which TWIST1 may mediate sorafenib resistance include reactivation/overexpression of the sorafenib target (i.e. PDGFR) or activation of bypass effector oncoproteins (i.e. HER3 or EphA3) in which a parallel signaling molecule activates downstream targets necessary for drug resistance. The experiments outlined in this F31 application aim to causally identify and mechanistically describe the pathways involved in TWIST1-induced sorafenib resistance and credential a novel STAT3-TWIST1 axis by which HCC tumors develop sorafenib resistance. First, STAT3 will be assessed for direct transcriptional activation of TWIST1 and the role of TWIST1 in sorafenib resistance. Secondly, candidate receptor tyrosine kinases will be analyzed to causally identify sorafenib resistance mechanisms. Thirdly, our novel TWIST1-inducible murine autochthonous liver tumor model will be utilized to evaluate TWIST1-induced sorafenib resistance in vivo and validate mechanisms identified with HCC cells in vitro.